In general, a vehicle equipped with an automatic transmission can automatically shift a transmission gear to a target gear shift stage by controlling a hydraulic pressure within a shift range set according to a driving speed of the vehicle and an opened amount of a throttle valve.
Therefore, in the vehicle equipped with the automatic transmission whose operation state is controlled according to the hydraulic pressure, it is not necessary to operate a clutch pedal for interrupting power from an engine in order to vary an operating state of the transmission gear. Thus, fatigue of a driver can be reduced, and the vehicle's engine does not stall during driving due to a false operation performed by the driver and driving inexperience. Therefore, even a beginner can easily perform a driving operation.
In the automatic transmission, a torque converter changes a rotational power of an engine, and a solenoid valve is controlled by a control signal applied from a transmission control unit (TCU) according to a driving state of the vehicle so that a hydraulic circuit which operates a friction element is formed. The hydraulic circuit executes an automatic gear shifting operation.
The automatic transmission outputs a control signal according to a position of a shift lever selected by the driver, the vehicle speed, and the opened amount of the throttle valve. The solenoid valve is driven by the output control signal so that port conversion of the valve in the hydraulic circuit is performed. Then, an operating state of the friction element for selecting one of gear shifting stages of a transmission gear mechanism is controlled by the hydraulic pressure supplied from an oil pump. An operating state of a planetary gear set is changed according to a selective operation of the friction element composed of a clutch and a brake so that the operating state of the planetary gear set is changed to an appropriate shift ratio. The appropriate shift ratio is transferred to a drive gear of a differential gear device.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.